1. Field of the Invention
This invention relates to a telecommunications device, particularly to a transceiver or the like that conducts wireless telecommunications using an adaptive array antenna or the like, still more particularly to a function for monitoring the transmit directivity of an adaptive array antenna used in a transceiver or the like.
2. Description of the Prior Art
Code division multiple access (CDMA) wireless telecommunications systems utilizing the adaptive array antenna (AAA) have come under consideration in recent years.
The adaptive array antenna assigns different weights (antenna weights) to multiple antennas so that signal reception and transmission can be conducted while controlling the directivity of the antennas as a whole. During reception, for example, the adaptive array antenna is controlled to maximize directivity in the direction of the desired incoming wave and strongly depress received signal quality with respect to signals from other directions. This control operation is conducted using a control algorithm provided in the device.
FIG. 9 shows the configuration of an AAA-equipped CDMA base station. As shown the CDMA base station incorporates an adaptive array antenna composed of four transmit-receive antennas A11–A14 (antenna #1–antenna #4), four transmit-receive wireless units W11–W14 each associated with one of the transmit-receive antennas A11–A14, and a common user-segregated AAA signal processor D2 for the four transmit-receive antennas A11–A14 and four transmit-receive wireless units W11–W14.
In the illustrated example, the total number N (N being a natural number) of antennas constituting the adaptive array antenna is four, and four transmit-receive antenna chains are constituted by the transmit-receive antennas A11–A14 and the four transmit-receive wireless units W11–W14.
An example of the operation of the illustrated CDMA base station will be explained.
The transmit-receive antennas A11–A14 of the CDMA base station receive signals wirelessly transmitted by one or more mobile stations and each outputs the received signals to the associated one of the transmit-receive wireless units W11–W14. The transmit-receive wireless units W11–W14 incorporate a function of converting (down-converting) the received signals coming in via the transmit-receive antennas A11–A14 from the radio frequency (RF) band to an intermediate frequency (IF) band or a base band (BB). The incoming signals are down-converted by this function and output to the user-segregated AAA signal processor D2.
The user-segregated AAA signal processor D2 CDMA-despreads the received signals coming in from the transmit-receive wireless units W11–W14 by user (e.g., by mobile station). Then, in order to implement the AAA function, it multiplies the despreading result for each transmit-receive antenna A11–A14 and each user by the receive antenna weights of the transmit-receive antennas A11–A14, synthesizes the multiplication results for all transmit-receive antennas A11–A14 by user and uses the results as the AAA receive signals for the individual users.
When receive antenna weights are determined by individual receive antennas using a given algorithm, the received signals from the receive antennas are multiplied by the receive antenna weights and the results are synthesized, an improvement in received signal quality can generally be obtained thanks to the receive directivity of the adaptive array antenna. By carrying out the same process in reverse, an improvement in transmitted signal quality can be achieved.
In a CDMA base station, for example, a common transmit signal for all transmit-receive antennas A11–A14 is prepared for each user (e.g., each mobile station), the user-segregated AAA signal processor D2 then generates for each user a different transmit antenna weight for every transmit-receive antenna A11–A14, multiplies each user's transmit signal by the transmit antenna weights of the individual transmit-receive antennas A11–A14, synthesizes the multiplication results for all transmit-receive antennas A11–A14, and defines the by-user results for all users as the AAA transmit signals of the transmit-receive antennas A11–A14. The transmission directivity of the adaptive array antenna attributable to the different transmitting power of the signals transmitted from the different transmit-receive antennas A11–A14 improves the reception quality from the viewpoint of the receiving parties (e.g., mobile stations).
Any of various algorithms can be used to generate the receive antenna weights and transmit antenna weights.
FIG. 9 shows a case where user data for X (X being a natural number) users are processed for reception or processed for transmission.
FIG. 10 shows an example of calculated antenna receive directivity patterns in an adaptive array antenna. The pattern designated (a) is an example in which the maximum directivity has been adjusted to 0 degrees for reception of an incoming wave from the 0-degree direction, and the pattern designated (b) is an example in which the maximum directivity has been adjusted to 45 degrees for reception of an incoming wave from the 45-degree direction. For reference, the drawing also shows the directivity component in the 180-direction that is the opposite direction from 0 degree, and the −45-degree direction that is the opposite direction from 45 degrees.
When received signals are processed with directivity controlled by such an adaptive array antenna, received signal processing can be conducted while eliminating interference entering the antenna owing to waves arriving from directions different from the arrival direction of the desired signal. The adaptive array antenna has therefore drawn considerable attention as a technology for eliminating interference.
The foregoing explanation regarding directivity during reception also applies to the directivity and interference removal effect during transmission, except that special measurement is required.
The special measurement required for an adaptive array antenna during transmission will now be explained.
This special measurement is called “calibration.” Calibration is for correcting error (deviation) in phase and amplitude that arises in transmission and reception chains including transmit-receive antennas and in the individual devices making up the chains owing to variation occurring during manufacture.
This will be explained with reference to a specific problem. Assume, for example, that based on a phase calculated from receiver output varying in phase and amplitude (gain), a mobile station is detected in the direction of 45 degrees as viewed from a base station. At transmission, the directivity of the adaptive array antenna should be adjusted to be maximum in the direction of 45 degrees. If the transmitter section has not been calibrated, however, it is actually impossible to impart high antenna directivity in the 45-degree direction because phase differences and level differences (amplitude differences) are present in the individual antenna chains. If the receiver section has not been calibrated, moreover, the detection of the mobile station in the 45-degree direction cannot be relied on from the start. The end result is that unless calibration is conducted the transmit directivity will be adjusted to a direction different from that where the mobile station is located.
FIG. 11 shows an example of the directivity patterns (e.g., transmit directivity pattern) achieved with an adaptive array antenna. Curve (a) represents a directivity pattern with calibration and curve (b) represents a directivity pattern without calibration. The horizontal axis of the graph is scaled for direction angle (degree) as viewed from the adaptive array antenna, and the vertical axis is scaled for the level [dB] of the transmitted/received signal. As the graph shows, when the communication antenna chains have been accurately calibrated, maximum directivity can be achieved in the direction of the angle V1 in which the mobile station is present (user direction), but when the communication antenna chains have not been accurately calibrated, the direction angle shifts owing to the calibration error, so that the direction angle of the maximum directivity becomes a different direction angle V2.
The communication antenna system calibration can, for instance, be receive calibration for correcting phase and amplitude deviation among transmit-receive antenna chains when receiving signals using the transmit-receive antenna chains, transmit calibration for correcting phase and amplitude deviation among transmit-receive antenna chains when transmitting signals using the transmit-receive antenna chains, and transmit-receive calibration for correcting phase and amplitude deviation between transmission and reception of a single transmit-receive antenna chain when receiving signals using the transmit-receive antenna chain and when receiving signals using the transmit-receive antenna chain.
The importance of communication antenna chain calibration is obvious from the attention it has received in, for instance, “Indoor transmission characteristics of adaptive antenna-array transmission diversity in W-CDMA downlink, Harada et al., Technical Report of The Institute of Electronics, Information and Communication Engineers, RCS99-18 (1999-05),” “Study on RF transmission and reception circuit calibration in W-CDMA downlink adaptive antenna-array transmission diversity, Harada, Tanaka, Sawabashi and Adachi, Technical Report of The Institute of Electronics, Information and Communication Engineers, RCS99-101 (1999-08)”, and “Automatic calibration method for FDD system adaptive array that takes antenna characteristics into account,” Nishimori, Osa, Takatori and Hori, Technical Report of The Institute of Electronics, Information and Communication Engineers, RCS99-213, MW99-233 (2000-02).
Conventional technologies related to calibration as discussed in the foregoing will now be reviewed.
In the phased array system taught by Unexamined Japanese Patent Application JP-A-8-248118, a monitor beam is transmitted from a phased array antenna toward a monitor beam antenna. When the level of the monitor beam received by the monitor beam antenna is not higher than a predefined reference level, the condition is judged abnormal and when the level of the monitor beam received by the monitor beam antenna is higher than the predefined reference level, the condition is judged to be normal. In other words, normal/abnormal condition is discriminated by determining whether the receive level is higher or lower than a reference level.
In the beam arrival direction detection method and system using the method taught by Unexamined Japanese Patent Application 2000-357911, a base station equipped with an adaptive array antenna uses an antenna chain installed independently of the adaptive array antenna to detect the directions of beams arriving from mobile stations.
In the wireless telecommunications system and wireless base station taught by Unexamined Japanese Patent Application 2001-7754, a terminal station determines the ratio between desired RF power transmitted to the terminal station from a base station equipped with an adaptive array antenna and interference RF power (D/U value) and transmits the determined value to the base station. The base station controls the weighting coefficient of each antenna device based on the determined value received from the terminal station.
In the method of regulating a transmit-receive system in an adaptive array base station and an adaptive array wireless system taught by Unexamined Japanese Patent Application 2001-53661, an adaptive array base station is configured to calculate the radio frequency amplitude and phase difference characteristics between the transmit system and receive system in each antenna and use the calculated calibration data to determine calibration that optimizes the phase and amplitude of the radio frequency transmitted from each antenna.
The problem indicated above with reference to the prior art of directivity pattern shift arising in transmit-receive antenna chains as a result of characteristic differences has already received the attention of adaptive array antenna researchers and is being overcome by calibration as explained in the foregoing. In the actual operation of commercial and other systems, however, problems such as those indicated by the following (1) to (4) can still be considered to occur.
(1) When a change occurs in a transmit-receive antenna itself or in the cable connecting the transmit-receive antenna to the base station owing to metallic expansion or contraction caused by temperature difference, such as between summer and winter, it becomes uncertain whether the error corrected by past calibration is identical to the current error.
(2) When the phase and amplitude (gain) characteristics of the transmit-receive antenna chain are markedly changed, such as by snow or wind, it becomes uncertain whether the calibration result keeps pace with the change in characteristics.
(3) A test conducted immediately before the first commercial operation does not provide certain proof that the calibration is working properly, so that doubt remains as to whether or not the error has actually been corrected by the calibration.
(4) When the central administrator managing a base station receives notification of trouble from the base station etc., it cannot easily discriminate whether the trouble is calibration related.
When any of the foregoing problems arises, reliability of the calibration operation can be assured only by having a technician from the company that built the equipment visit each site to confirm whether the calibration operation is working properly. This increases the cost of running a base station or the like equipped with adaptive array antenna capability and also complicates supervision work. Another inconvenience is that a person without expert technical knowledge is unable to ascertain whether the state of calibration operation is normal or abnormal.
The present invention was accomplished for overcoming such disadvantages of the prior art and has as its object to provide a telecommunications device that, in a system for conducting wireless telecommunications using an adaptive array antenna, enables monitoring of the transmit directivity of the adaptive array antenna, thereby ensuring highly reliable transmit directivity of the adaptive array antenna.